The present invention concerns a heat exchanger. Heat exchanger tubes are concentrated in at least two horizontally arranged bundles, with heating steam flowing within the tubes and with a power medium, to be heated up, flowing around the tubes. The heating steam flows serially through the bundles or nests of tubes and the power medium flows serially around the nests of tubes. The several bundles are connected upstream with an inlet manifold and downstream with a drainable collecting chamber. The several nests of tubes possess different heat-exchanging surfaces which are arranged such that the cross-sectional area of the nests of tubes, through which the heating steam is flowing, will decrease in direction of the heating steam flow. Steam-heated heat exchangers of this type, used for example to superheat exhaust steam of high-pressure turbines in nuclearly heated saturated steam turbine plants, where the heat-dissipating heating steam is condensed in the course of several pass-throughs in order to attain a maximum thermal flow rate and to maintain safety of operations, are known (see published German patent application No. 22 00 916). At each pass-through there is being condensed only such quantity of heating steam that, even in the case of the most disadvantageously placed tube, the steam/condensed-water flow at the tube end will be free of instabilities which could cause periodic fluctuations in the temperature of the tube wall and thus permanent damages of the tubes or the joint between tube and tube base. The condensed water is removed from the heating steam when it commences its next pass-through in order to facilitate thermal dissipation and to avoid pressure losses at the flow through the next bundle of tubes. Heat exchangers of this type can be formed by straight-line tubes or by U-shaped tubes, the latter offering the advantage that slight differences in thermal elongation of the tubes can be controlled with greater ease.
It has been proposed to employ pin-hole plates, mounted at the intake end of each nest of tubes, for a precise throttling of the heating steam. Such pin-hole plates have the disadvantage that a seal between the individual banks of tubes can not be attained because the beads of the tube welds will protrude in an irregular manner so that unwanted by-passes will be formed. It is further necessary to attach the plate in such manner that it will be able to move because inadmissibly high thermal stresses would be generated otherwise. Finally, the plate is so large that it can not be removed in one piece when the steam chamber has been welded together.
Tube sections, introduced into the intermediate superheater tubes at their intake side, have also been used. Each tube section is divided at its longitudinal center and a pinhole diaphragm is welded into this spot. This arrangement has the disadvantage that the insert tube section can not be readily removed after its installation, making it impossible for all practical purposes to inspect the tube inside. Furthermore, the manufacture of such tube sections is costly.
It is the object of the present invention to establish a throttling arrangement for a heat exchanger which will make possible a reduction in the number of heating steam pass-throughs without affecting the safety of operation, namely by adjusting the flow rate of the heating steam in functional relation to the thermal load of the various tubes. The manufacture of the throttling arrangement should be simple and inexpensive, and the assembly as well as the disassembly of the throttling arrangement should be possible without any difficulties.
The invention solves this problem by providing, at the intake openings of the tubes through which the heating steam is flowing, baffles with different intake profiles. Within each nest of tubes the intake profiles become smaller in a direction of flow of the power medium to be heated.
The frontal arrangement of baffles makes it possible to relate the flow rate of the heating steam precisely to the thermal load (.DELTA.T) of each tube so that the scavenging steam rate at the tube will correspond to the minimum rate of flow which is required. Since the over-all pressure losses for the condensation process are very low, the thermodynamic loss caused by the throttling will be so low that it can be disregarded. This arrangement makes it also possible to keep the number of heating steam pass-throughs to a minimum without affecting the safety of operation of the aggregate by relating the flow rate of the heating steam to the existing thermal load of the tubes.
An advantageous further development of the invention object provides that the baffles are formed by a slotted, cylindrical baffle body with a collar defining the baffle opening. The collar's outer diameter is greater than the intake opening of the tube.
It is also preferable to provide the surface of the baffle body with a tapered trailing edge in order to eliminate any flow separation. The outer surface of the baffle body can further be provided with an eccentric relief adjacent to the collar.
The placement of the baffle in front of the tube intake eliminates the need for a calm region in front of the baffle, and a more precise balancing of the pressure drop based on a number of flow-throughs unaffected by the steam flow velocity will facilitate the layout of the heat exchanger. The slotted, cylindrical form of the baffle body permits, due to its inherent elasticity, an equalization of differences in thermal expansion. The simple geometry of the baffle bodies makes it possible to manufacture such bodies precisely and economically. The baffle bodies of the present invention can be installed in a simple manner by driving them into the tube intake openings. A correspondingly simple disassembly allows an inspection of the tube inside and of the joint connecting the tube with the tube base without costly prior preparations. This baffle system also permits a quick adjustment in response to changes in operating conditions when scavenging steam is present in excessive or insufficient quantities.